The operating principle of proton exchange membrane fuel cells (PEMFC) is based on the conversion of chemical energy into electric energy by catalytic reaction between the fuel (hydrogen) and the oxidizer (oxygen).
Membrane-electrode assemblies (MEAs), commonly called cell cores, form the base elements of PEMFCs. As illustrated in FIG. 1, the MEA is generally formed of a polymer membrane (electrolyte, 3) in contact with a catalytic layer (electrodes, 2) on both sides. The electrodes (anode and cathode) are thus separated by the electrolyte, which is insulating for electrons but however conducts protons. Current collectors (1) ensure the electron transfer at the external surface of the electrodes.
In the case of proton exchange membrane fuel cells, the electrolyte generally is a membrane formed of a cation exchange polymer, such as Nafion® (Dupont) or Aquivion® (Solvay).
The fuel used in proton exchange membrane fuel cells may be a gas such as hydrogen or a liquid, such as for example, an alcohol (ethanol, methanol, ethylene glycol).
For example, the following reactions illustrate the electrochemical reactions occurring at the electrodes in the case where the fuel and the oxidizer respectively are hydrogen and oxygen:Anode: H2→>2H++2e−  (1)Cathode: O2+4H++4e−→2H2O  (2)Eoanode=0V/ENH Eocathode=1.23V/ENH 
In this case, the global reaction thus is the following:H2+½O2→H2OEoeq=Eocathode−Eoanode=1.23 V
The electromotive force across the cell thus is 1.23 V in standard conditions.
At the anode, the decomposition of the hydrogen adsorbed on the catalyst generates protons H+ and electrons e−. The protons then cross the polymer membrane before reacting with oxygen at the cathode. The reaction of protons with oxygen at the cathode results in the forming of water and in the generation of heat.
Such electrochemical reactions are kinetically promoted by the presence of a catalyst forming the electrodes. Several materials may be used according to the type of reaction and of fuel, but platinum appears to be the most effective catalyst for most reactions and fuels. As already indicated, the catalyst may appear in the form of catalytic layers which are generally made of platinum nanoparticles supported on carbon aggregates.
The catalyst may be uniformly deposited by means of a catalytic ink on the membrane surface or on the diffusion layer. Such a catalytic ink is especially formed of the catalyst supported by carbon (platinized carbon), a carrier liquid, and a proton conductive polymer. The latter is generally of same nature as the electrolyte.
Document JP 2005174768 describes a composition for an electrode comprising a platinum-based catalyst, Nafion®, and carbon black.
Further, document EP 0945910 describes an electrode comprising a catalyst based on platinized carbon and an ionomer. The described active layer comprises from 0.01 to 5 mg of metal/cm2 while the pore volume is from 0.7 to 1.3 ml/g.
Document JP 92835154 describes an electrode composition comprising an ionomer and platinized carbon. Further, this document specifies that it is preferably for pores, having a diameter greater than 0.1 μm, to have a volume at least equal to 0.4 cm3/g.
With a view to the use and the development of PEMFCs for the consumer market, it is essential to minimize phenomena of aging of the core of such cells.
The liquid water flooding of the cathode and the irreversible degradation of the cathode nanomaterials (carbon support and catalyst) are among the main phenomena taking part in the degradation of the cell performance.
The presence of water is essential for the proper operation of the PEMFC since it especially enables to maintain the conductivity of the proton conductive polymer present in the electrodes and in the membrane. It is generated at the cathode according to reaction (2) but may also be introduced by previous humidification of the gases. However, an excessive amount of water is prejudicial since it may cause the flooding of catalytic sites and thus a stopping of the cell by making the access of the gases to the reactive sites impossible.
Further, the irreversible degradation of the electrode materials essentially concerns the active cathode layer according to a well-known corrosion mechanism, particularly prejudicial for the cell. The catalytic carbon support at the cathode oxidizes according to the following reaction:C+2H2CO2+4H++4e−  (3)
Reaction (3) is generally thermodynamically possible, given its low potential (0.2 V/ENH) with respect to the cathode potential of a cell. It is promoted by the presence of liquid water.
This degradation is enhanced when the cell is submitted to power cycles (J. P. Meyers and R. M. Darling J. Electrochem. Soc., 153 (8), A1432, 2006).
On the other hand, the oxygen present at the anode is normally reduced by hydrogen in the anode compartment. However, the hydrogen which is present may be insufficient in stop/start phases, power cycles, the forming of water plugs, the stopping of the hydrogen supply. The oxygen then uses other proton sources and, in particular, those generated by the oxidation of the cathode carbon. The oxygen present at the anode thus acts as a proton pump and enhances the corrosion of carbon at the cathode.
The degradation of platinum at the cathode also contributes to decreasing the cell performances. One of the degradation mechanisms relates to the oxidation, the dissolution, and the recrystallization of platinum.
On the other hand, the degradation of the cathode carbon support may cause the separation of the platinum particles and go along with a decrease in the active surface area due to the agglomeration of platinum particles (A. A. Franco and M. Gérard J. Electrochem. Soc., 155 (4), B367, 2008; Y. Shao, G. Yin, and Y. Gao J. Power Sources., 171, 558, 2007). It is proportional to the density of platinum nanoparticles supported on its surface (A. A. Franco and M. Gérard J. Electrochem. Soc., 155 (4), B367, 2008).
Electrochemical aging is another platinum degradation mechanism which results in an increase in the size of platinum nanoparticles and thus in a decrease in the electroactive surface area and of the cell performances. The electrochemical aging is enhanced when the amount of local water around the nanoparticles increases, or when the distance between nanoparticle decreases, or when the particle size dispersion increases.
Various solutions have been provided to decrease the performance degradation of a PEMFC-type fuel cell during its operation.
The decrease or the suppression of the carbon corrosion at the cathode may thus be achieved by modification of the nature of the support carbon, by addition of carbon dioxide in the oxidizer gas (cathode), by introduction of carbon monoxide in the fuel gas (anode), or by using a protection layer.
The modification of the nature of the support carbon may be performed by using carbon supports more resistant to corrosion, such as carbon nanotubes, fullerenes, graphites, or by thermal processing of the carbon support.
The addition of carbon dioxide (CO2) in the oxidizer gas (oxygen) at the cathode enables to displace the equilibrium of the reaction (3) to the left to thus slow down the carbon corrosion.
The introduction of carbon monoxide (CO) at the anode enables to limit the carbon corrosion at the cathode. Indeed, by reacting with the oxygen present at the anode, the CO limits the above-described “proton pump” (A. A. Franco, M. Guinard. B. Barthe, O. Lemaire, Electrochimica Acta, 54 (22) (2009) 5267).
The use of a non-conductive silica-based protection layer (SiO2) on a carbon/platinum nanotube assembly has been provided (S. Takenaka, H. Matsumori, H. Matsune, E. Tanabe, and M. Kishida, J. Electrochem. Soc., 155 (9) (2008) B929). This layer aims at limiting the migration of platinum, and thus the forming of catalytic particle aggregates responsible for a decrease of the cell performance. This solution thus acts on the stability of the catalyst, but not on the carbon, which is already stable. It should be reminded that carbon nanotubes have a good stability regarding the corrosion of carbon. Further, the SiO2 layer is formed on all the carbon/platinum nanotube systems by hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane compounds previously mixed to the carbon/platinum nanotube systems.
Another way to decrease the degradation of PEMFCs comprises decreasing the cathode flooding phenomenon. Although the volume of the active layer can be decreased by depositing less platinized-carbon-based ink and thus less catalyst, this increases the cathode sensitivity to flooding. Several solutions have been provided to overcome this phenomenon.
Document U.S. Pat. No. 6,492,295 provides the deposition of hydrophobic resins at the surface of the catalyst particles to limit the forming of liquid water in the cathode.
Document U.S. Pat. No. 5,723,173 provides introducing into the active layer a carbon powder having previously been submitted to a hydrophobic processing based on the fluorocarbon polymer.
On the other hand, document US 2008/0090128 provides using a catalytic powder which, after having been dispersed in water (0.5 g for 20 g of water), has a pH at least equal to 6. This pH range makes the powder present in the active layer relatively hydrophobic.
Document US 2008/0044697 provides forming an active layer which is extremely resistant to wetting, by the introduction of silicon compounds.
The introduction of a hydrophobic oil, namely polydimethysiloxane, into the electrode pores has also been provided (M. B. Ji, Z. D. Wei, S. G. Chen and L. Li, J. Phys. Chem. C 2009, 113, 765-771).
All these solutions aim at increasing the hydrophobic character of the active layer. However, such processings may be subject to chemical degradation mechanisms after several hundreds of hours of use. Further, the introduction of an additional product may decrease the electrode porosity, and thus limit the diffusion of oxygen. The introduction of hydrophobic materials may further cause the absence of water close to the catalyst while its presence is necessary for electrocatalytic reactions, as well as for proton conduction.
The present invention overcomes all these disadvantages by providing a structuring of the electrode to limit the reversible and irreversible degradation of a cathode.
The flooding phenomenon and the carbon corrosion and electrochemical aging phenomena are thus decreased.